Electrochemical cells, such as fuel cells, consume reactant gases to electrochemically produce a reactant product and electrical power. Waste heat is a by-product of the electrochemical reaction. A cooling system is provided for removing the waste heat to maintain the temperature of all cells at a uniform level which is consistent with the properties of the material used in the fuel cell and the operating characteristics of the fuel cell.
Examples of cooling systems used in electrochemical fuel cells are shown in the following issued patents, the material in which is incorporated herein by reference: U.S. Pat. No. 4,245,009 issued to Guthrie entitled "Porous Coolant Tube Holder for Fuel Cell Stack"; U.S. Pat. No. 3,969,145 issued to Grevstad et alium entitled "Fuel Cell Cooling System Using A Non-dielectric Coolant"; U.S. Pat. No. 4,233,369 issued to Breault et alia entitled "Fuel Cell Cooler Assembly and Edge Seal Means Therefor" and U.S. Pat. No. 4,269,642 issued to De Casperis et alia.
As shown in U.S. Pat. No. 4,233,369 issued to Breault et alia, cooler assemblies are disposed in the fuel cell stack for cooling the stack. Cooling fluid from a supply chamber is supplied to the cooler assemblies by a feed tube. The cooling fluid is returned from the cooler assemblies to the supply chamber via a return tube. An inlet header is in flow communication with the feed tube and an outlet header is in flow communication with the return tube. A plurality of cooling tubes at each cooler assembly extend in parallel between the inlet header and the outlet header and are disposed in the cooler assembly. The cooler assembly is adapted by channels to receive the cooling tubes.
The outlet of power by the stack may be increased by adding electrochemical cells to the stack. The additional cells increase the length of the cell stack. As the length of the cell stack increases, the feed tube and return tube are lengthened to bring cooling fluid to the additional cooler assemblies. The increase in the length of the tubes causes an increase in the loss of pressure experienced by the fluid as the fluid flows between the first header and the last header. Proper sizing of the feed and return tubes can produce approximately equal overall pressure losses in both tubes. As the tubes become longer, the different flow characteristics in each tube results in unequal pressure gradients over some regions and causes, in some cases, an inadequate amount of flow to some headers and an excessive amount of flow to other headers. This condition of unequal flows between headers is referred to as flow maldistribution. Cooler to cooler flow maldistribution can also result from variations in head loads between cooler assemblies as a result of variations in cell performance and as a result of variations in flow cross-sectional areas due to the deposition of dissolved species and suspended particulates. Flow maldistribution can also occur within a cooler whose tubes are aligned in a parallel flow configuration as described above due to variations in local heat loads which results from variations in current density across the cell.
One approach to solving the problem of flow maldistribution is to increase the field resistance in the assembly, that is the flow resistance between the center of the feed tube and the center of the return tube, such that variations in flow resistance between the first header and the last header are insignificant in comparison to the field resistance. Flow resistance is increased, for example, by an orifice which has a diameter which is much smaller than the diameter of the tubes. Recent experiences have shown that such small orifices are prone to plugging in systems using a coolant, such as water because of dissolved species or particles in the coolant which form deposits on the walls of the orifices. One solution is to provide a cooling fluid which is treated to remove the dissolved species and particles. Nevertheless, there are many situations in which the cleanliness of such coolant is limited by economic or physical reasons.
Accordingly, a requirement exists for a cooling system which avoids the problem of plugging and, at the same time, avoids the problem of flow maldistribution between cooler assemblies.